Comet Goldfish (Carassius auratus var.)

The Comet Goldfish is a single-tailed goldfish variety developed in the United States in the 1880s from the Common Goldfish (Carassius auratus). Its defining characteristic is the long, deeply forked caudal fin that streams behind the fish as it swims — giving the impression of a comet's tail. The Comet is the most streamlined and fastest-swimming of the common fancy goldfish varieties.

Comet Goldfish are available in a range of colours including orange, red, yellow, white, and combinations thereof. The 'Sarasa Comet' — a red and white colour variety — is one of the most popular. All are robust, cold-water fish that do not require heaters in European climates.

Goldfish are cold-water cyprinids that produce enormous amounts of waste relative to their size — significantly more than tropical fish. This bioload demands excellent, high-capacity filtration and regular, substantial water changes. The misconception that goldfish can thrive in small bowls or minimal conditions has caused untold suffering and early death for countless goldfish over the decades.

For optimal health and to reach their full potential (30+ cm in ponds), Comet Goldfish are best suited to outdoor ponds or very large indoor aquariums (400+ litres). A school of six juvenile Comets will rapidly outgrow a 200-litre tank. In appropriate conditions, Comets are extremely hardy and can live 15–20 years or more.

Tank Setup: Minimum 200 litres for a small group of two to three juveniles — more as they grow. A pond is strongly preferred for adult Comets. High-capacity filtration is essential (turn over 5–10x the tank volume per hour). Cold water — no heater required in Belgium; indeed, goldfish prefer cool water and suffer in warm tropical conditions.

Water Parameters: Temperature 10–22°C; pH 7.0–8.0; GH 8–20 °dH; KH 5–12 °dH. Cold-water species — do not heat. Prefer slightly hard, alkaline water. Excellent oxygenation is critical due to their high oxygen demand.

Feeding: Omnivore. High-quality goldfish pellets or flake (wheat germ-based for cold weather), blanched vegetables, Daphnia, and occasional live foods. Feed once to twice daily — only what is consumed in 2–3 minutes. Overfeeding is the primary cause of water quality problems. Reduce feeding significantly in winter.

Tank Mates: Best with other goldfish: Comets, Common Goldfish, Shubunkins. Cannot be kept with tropical fish due to temperature incompatibility. Avoid fancy double-tailed goldfish (Orandas, Ryukins) — they swim too slowly to compete for food.

Behaviour: Fast, active swimmers that explore all levels of the tank or pond. Social — prefer companions. Cohabitate peacefully with other goldfish. Dig in substrate — avoid delicate plants.

Health: Hardy under correct conditions. Main issues are ammonia and nitrite accumulation from inadequate filtration. Ich and bacterial infections are secondary concerns. Goldfish kept in warm or small tanks have dramatically shortened lifespans.